1. Field of the Invention
The invention at hand relates to the field of power electronics. It concerns a method for protecting a high-performance power converter, the high-performance power converter comprising a mains power converter which converts an alternating voltage from a mains power supply into a direct voltage and supplies if. to an intermediate direct voltage circuit equipped with intermediate circuit capacitor, and comprising a second power converter which converts the direct voltage from the intermediate direct voltage circuit into an alternating voltage, whereby each of the power converters is set up as a bridge circuit and each contains a number of IGBTs (Insulated Gate Bipolar Transistors) arranged in pairs in bridge arms.
2. Background Information
High-performance power converters comprising two power converters connected with each other via an intermediate direct voltage circuit have been used for some time for motor drives in electric trains, but also for other applications. The first power converter functions as a mains converter and converts the alternating voltage with a first frequency taken from an alternating voltage supply into a direct voltage that is supplied to the intermediate direct voltage circuit. The second power converter, the motor power converter, converts the direct voltage from the intermediate direct voltage circuit into an alternating voltage with a second, also variable, frequency, which is then used to supply the motor. Such alternating voltage motor drives are known, for example, from U.S. Pat. No. 4,959,602.
Both the mains power converter and the motor power converter are usually equipped with gate turn-off power semiconductors (GTOs or IGBTs) arranged in pairs in bridge aims and connected to form a bridge circuit. An example of a motor power converter equipped with GTOs (Gate Turn-Off thyristors) is described in U.S. Pat. No. 4,672,525. An example of a motor power converter equipped with IGBTs is found in U.S. Pat. No. 5,592,371.
During normal operation, the switch pairs in the bridge arms of the power converters are never switched through at the same time. But during malfunctions both switches of a bridge arm pair may become unintentionally continuously conductive. The bridge arm then represents a short circuit for the supplied direct voltage, resulting in a high short circuit current through the bridge arm. To manage this malfunction, the above mentioned publication U.S. Pat. No. 4,672,525 suggests for a motor power converter driven from a direct voltage supply that the motor power converter's bridge arms, equipped with GTOs, be monitored individually for a short circuit (switching through of both GTOs of the arm), and in the case of a short circuit in one bridge arm, to also specifically switch through the remaining bridge arms.
Such a safety break-through causes the flowing short circuit power to be distributed over several bridge arms and switching elements, making it possible to avoid damage to the individual components (GTOs) during the initial time of the malfunction. The power converter is then separated from the direct voltage by a successive opening of a "high-speed breaker" and a (relatively inert) mechanical switch, and the short circuit power supplied by the direct voltage source can be permanently interrupted in this way (safety cutoff).
In a high-performance power converter with intermediate direct voltage circuit and IGBT as electronic switches, as the one used as a basis for this invention, a protection concept with a safety cutoff is also advantageously used due to the characteristic properties of the IGBTs. If an unintentional simultaneous conduction of the electronic switches of an arm pair occurs within such a power converter, first the intermediate circuit capacitor discharges partially or completely over the protection elements switched in series with it, and in this way more or less destroys the electronic switches. After such a burst-like discharge of the intermediate circuit during a range of about one millisecond, the intermediate circuit is short-circuited, but the intermediate circuit capacitors may not yet have been completely discharged depending on the type of the protection elements switched in series with them. The short circuit currents still flowing at this point via the still intact power converter arm pairs from the mains and the motor (the load) into the short-circuited and partially destroyed arm pair are able to significantly increase the extent of this arm pair's damage and may cause serious damage near this arm pair.